Superfine fibers of a thermoplastic polymer such as polyester, nylon or the like have been used for the production of products having high added value. Particularly, fibers of not more than 0.5 monofilament denier have been used for the production of artificial leather, high-class clothes and the like.
Upon producing such superfine fibers, in general, a molten thermoplastic polymer is extruded from a spinneret, the extrudate is quenched by cooling air flowing in a direction across the extrudate, and then the extrudate is stretched to obtain multifilaments. As described above, the superfine fibers are requested to have not more than 0.5 monofilament denier. On the other hand, the fineness of multifilament yarns made of the monofilaments is requested to be not less than 20 denier like normal filament yarns. Therefore, it is required to use a spinneret having a lot of nozzle orifices in the production of superfine fibers. Then, quenching of filaments with the above cooling air tends to become ununiform and physical properties of respective filaments vary, which causes trouble such as filament breaking or the like, frequently. This is a significant problem from the operational viewpoint.
Then, various studies have been done to solve this problem. For example, JP-A 54-64119, JP-A 54-73915, JP-A 54-30924 and JP-A 54-88316 disclose technique for improving spinning stability from the viewpoints of a diameter of a nozzle orifice bored through a spinneret, an extrusion rate, a density of orifices, a minimum orifice interval, a wind-up rate and the like.
However, when the number of orifices of a spinneret is increased according to the above technique, difference in solidification of filaments by quenching is caused at orifices located in a leeward side of cooling air (hereinafter referred to as counter-quench side) among the orifices arranged on the nozzle and, therefore, filament breaking or the like during stretching is caused due to variation of crystallinity or orientation. Even if filament breaking is not caused, difference in physical properties of filaments is caused due to difference in the above quenching conditions between orifices located in a windward side of cooling air (hereinafter referred to as quench side) and the counter-quench side, which results in the cause of trouble not only in the spinning step but also in subsequent steps.